1. Field
The subject matter disclosed herein relates to data communication systems. In particular, the subject matter disclosed herein relates to transmitting data in an optical transmission medium.
2. Information
Data transmission in an optical transmission medium such as fiber optic cabling has enabled communication at data rates of 10 gigabits per second and beyond according to data transmission standards set forth in IEEE P802.3ae and Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) published by the International Telecommunications Union (ITU). To transmit data in the optical transmission medium, a laser device typically modulates an optical signal using wave division multiplexing (WDM) in response to a data signal.
FIG. 1 shows a schematic diagram of a prior art laser driver circuit 50 to provide a modulation current 60 to a laser device 58. The laser device circuit may be formed in a single complementary metal oxide semiconductor (CMOS) device. The laser device 58 receives a modulation current 60 to power the transmission of an optical signal 62 in an optical transmission medium. The laser device 58 typically also modulates the optical signal in response to a data signal. The laser driver circuit receives a reference current 52 generated by, for example, a controlled voltage source applied across an off-chip resistor. A diode coupled field effect transistor (FET) 54 and FET 56 form a current mirror to generate the modulation current 60 at a magnitude that is substantially proportional to the magnitude of the input reference current 52. Short channel lengths in such a CMOS device may introduce channel length modulation effects, causing nonlinearities in the output modulation current 60 (e.g., affecting the ratio of the reference current 52 to the modulation current 60). The resulting magnitude of the modulation current 60, therefore, may not be sufficiently accurately generated for high data rate communication in an optical transmission medium.